boldI'm tired of my T4i's mediocre detail and color balance for video and its lousy photo performance. So I'm looking to get a new body on eBay. I have a bunch of all lenses and STM lenses, so I'm trying to figure out what body to get.
The T4i was not made to be the best of the best; that is why Canon introduced it as beginner DSLR in their Rebel line. What type of glass do you have a bunch of if I may ask?
boldI do mostly impromptu press conferences and studio filming, so I need good autofocus but strong low light performance. I have a 7D for the studio shots, but it's terrible for impromptu press conferences. The 70D seems ideal for autofocus, but I assume will be inferior in picture quality to the full frames. Any advice on whether the servo autofocus on the full frames is good enough to pass up the 70D.
From what I'm guessing you're asking is that: you're looking for the best of both worlds which means fast AF, best high ISO performance, can use in a variety of settings from filming to still photography to press photography. I rarely shoot in AI Servo since I do not do much with BiF/Sports/Wildlife so I don't necessarily have a say in that. But, your options would currently be the 70D OR 7D Mark II OR 6D OR 5D Mark III.
boldI'm buying off eBay with a budget of around $600, so my options seem to be the 70D or the 6D. But I'll pay extra for the 5D Mk III if it's that much better for video than the 6D. There is also the T6i body new, which I assume is in my range. But I want much better image quality than the T4i and I assume the T6i isn't noticeably better like a full frame would be.
Now here is where your trouble comes in: 1. Your budget is $600 which is no where close to any of the bodies you're trying to get your hands on other than the 70D (which you might be able to find under $750 if you're lucky and I am talking BRAND NEW, not USED) 2. You will NOT find a Canon 6D for $600. It has not even broke past the $1000 range yet on eBay retailers. 3. The Canon 6D is in it's own class as the first budget full frame DSLR from Canon. I do not know too much about the video side of the Canon 6D, but I know the image quality of the 6D is well worth every penny you will currently pay for it. People are trying to wait for the Canon 6D Mark II. However, that is all but a rumor and you will be waiting a longer time for it and possibly paying more than $2000 for one if it does come out within the next 2-3 years. 4. The 5D Mark III will still be way out of your budget if you're looking to spend a little more to purchase the 5D Mark III. It's a remarkable full frame DSLR in its' own very class and that is why many professional photographers use it for all sorts of photography as well as videography. All in all, for the image quality you're looking for, the beginner Rebel line cannot be on par with the xD line of Canon DSLR. If you want the best of the best, then put your money into a 1DX and you won't have to decide from there.
All I'll say is that nothing is going to be the best, the Canon 7D Mark II would be great for what you do; impromptu press conferences, filming (videographers do also us the 7D Mark II to film). The 6D is a full frame and the image quality is fantastic for being almost a third of the price of what you'd pay for a 5D Mark III. Except one down side is that the AF system sucks ball with only 11-point AF and 1 cross-type AF; even then, the 6D's AF is rated to a -3EV and so low-light focus will not be a problem. The 5D Mark III, like I said, is in it's very own league and it's just an overall great DSLR for any type of photography other than sports, wildlife, and bird-in-flight (it's decent, but that's where the 7D Mark II and 1DX will outperform the 5D Mark III on track focusing).
Anything that you buy to upgrade from the T4i, regardless it be the 70D/7D Mark II/6D/5D Mark III will be better than the T4i. All that is stopping you is the amount of money you can spend on your new body.
Cheers!